Description of the general prior art with respect to stress control can be found, for example, in the German Offenlegungsschrift No. 28 21 017.
Materials of the general kind indicated above are known, for example, from U.S. Pat. No. 4,053,702. They contain titanium dioxide as the effect material. That known substance makes possible, inter alia, the manufacture of permanently resilient stress control elements of definite geometric configuration, which may be simply shifted on, while yielding resiliently, at the site of application, typically a cable connection. Due to their resilience, they then fit in a gap-free manner. That strong and gap-free fitting is retained, due to the permanently resilient properties, over very long time periods, for instance many years, and particularly over the usual operational life of power/current systems. The application of such permanently resilient stress control elements requires less knowledge and skill than the application of other stress control devices, such as, for example, metallic stress control cones, the gap-free wrapping of tapes of stress-controlling material, the molding or modelling and subsequent hardening of flowable or shapeable masses having stress-controlling properties at the site of operation, etc.
Materials of the kind described initially therefore have made possible a considerable advance in the field of stress control in power current and high voltage systems.
The materials described initially act, together with cable insulating materials of low dielectric constant, upon electric fields in the sense of a refraction. For the sake of completeness, it should be mentioned that for the manufacture of stress-controlling devices, other materials are known which mainly act in a resistive manner; they contain electrically conductive or semiconductive effect materials which provide to the material a desired (mostly voltage-dependent) electrical conductivity (U.S. Pat. Nos. 3,673,305; 3,666,876). In these cases, permittivity is also increased by the embedding of particles of electrically conductive or semi-conductive effect material; for example, it may be up to 11 (U.S. Pat. No. 3,666,876). With resilient materials having resistive stress control properties, however, the active current, which due to the function flows continuously, may gradually give rise to changes of electrical conductivity, and to a premature ageing of the material. Thus, other modes of operation are preferred for permanently resilient materials, particularly the refractive mode of operation which also applied to the material of the present invention.
It will be appreciated that a relatively high permittivity of the material is desirable. Thus, it is desired to use substances having a permittivity as high as possible as the effect material for materials showing refractive stress control action, e.g. the titanium dioxide already mentioned, but also other known substances of very high permittivity, e.g. barium titanate. The use of such materials as an effect material for materials having a refractive stress-controlling action has been known for a long time, but without paying particular attention to the requirement of permanent resilience (U.S. Pat. Nos. 3,673,305; 3,823,334; 3,287,489). In this connection, however, it was found also that when utilizing effect materials of very high permittivity, e.g. barium titanate having permittivities of approximately 10,000, the permittivity of the material cannot be increased beyond approximately 25 if the material is to retain the permanently resilient properties of the base material. The reason for this therefore is that in mixtures of that kind, the permittivity .SIGMA..sub.r mix of the mixture has to be calculated according to a logarithmic, and not according to an additive, mixture formula from the permittivities .SIGMA..sub.rn of the components of the mixture: ##EQU1## in which X.sub.n is the volume ratio, and .SIGMA..sub.rn is the relative dielectric constant (permittivity) of the component .eta.. Accordingly, that formula shows particularly that with a material consisting of two components, namely,
Component 1: Elastomeric base material having an .SIGMA..sub.r of about 3, PA1 Component 2: Barium titanate having an .SIGMA..sub.r of about 10,000, PA1 (a) The volume resistivity has at least a minimum value which is still sufficient for purposes of electrical insulation, PA1 (b) The relative dielectric constant (permittivity) is greater than 30, and up to about 300, and PA1 (c) The dielectric loss factor is not greater than approximately 1.5
one would have to employ a proportion of about 35 volume percent or about 75 weight percent barium titanate to obtain a permittivity of the mixture of about 50. With the proportion of effect material being so high, the resilient properties, however, of the material are insufficient to permit the manufacturing therefrom of practicable gap-free fitting stress control elements having permanent resilience. The resilience itself, as well as the maintenance in time of an elastic tension once produced in the material (the so-called permanent resetting force), are insufficient. It was generally true for the prior art (e.g., U.S. Pat. No. 4,053,702) that permanently resilient materials of the kind initially described could be manufactured with permittivities of only up to about 25, even when employing the known effect materials having very high permittivities. The permanent resiliency of dielectric materials can only be observed and maintained for the technical practice of testing results achieved if the residual stress (permanent set) under constant deflection according to the specification DIN No. 53517 (ISO-STANDARD R 815-69), particularly after accelerated ageing for 72 hours at 150.degree. C. are always less than about 35% and results of SHORE-A Hardness according to the specification DIN No. 53505 (ISO-STANDARD R 868-68) are less than 65. If these requirements characterizing permanent resiliency cannot be met it cannot be warranted that such materials can be used to produce elastic stress control elements. All described materials known until now, which have a permittivity above 25 do not pass this important criteria of permanent resiliency.